A motor unit consists of:

• A. Several motor neurons connected to a single muscle fiber
• B. A single motor neuron and all the muscle fibers it innervates
• C. A muscle fiber and all the neurons that supply it
• D. All the muscles in a given muscle group

Answer: (B)

Verified step by step guidance

1. Understand the definition of a motor unit: A motor unit is the functional unit of the neuromuscular system, consisting of a single motor neuron and all the muscle fibers it innervates.
2. Clarify the role of the motor neuron: A motor neuron is a nerve cell that transmits signals from the central nervous system to muscle fibers, causing them to contract.
3. Recognize the relationship between the motor neuron and muscle fibers: A single motor neuron can innervate multiple muscle fibers, but each muscle fiber is innervated by only one motor neuron.
4. Eliminate incorrect options: For example, 'several motor neurons connected to a single muscle fiber' is incorrect because a single muscle fiber is innervated by only one motor neuron. Similarly, 'a muscle fiber and all the neurons that supply it' is incorrect for the same reason.
5. Conclude with the correct answer: A motor unit is defined as a single motor neuron and all the muscle fibers it innervates, which matches the correct option provided in the problem.